Through-the-wall-imaging (TWI) can be used to detect objects inside enclosed structures. In TWI, a transmitter emits an electromagnetic (EM) radar pulse, which propagates through a wall. The pulse is reflected by a target on the other side of the wall, and then propagates back to a receiver as an impulse response convolved with the emitted pulse. Typically, the transmitter and receiver use an antenna array.
Depending on a dielectric permittivity and permeability of the wall, the received signal is often corrupted with indirect, secondary reflections from the wall, which result in ghost artifacts in an image that appear as noise. Wall clutter mitigation techniques attempt to eliminate the artifacts that arise from the multi-path reflections in TWI.
Some methods derive multi-path signal models to associate and map the multi-path ghosts to target locations. In a physics based approach to multi-path exploitation, an imaging kernel of a back-projection method is designed to focus specific propagation paths of interest. Target sparsity in TWI systems has also been used for multi-path elimination, specifically in compressive sensing synthetic aperture radar (SAR). That approach incorporates sources of multi-path reflections of interest into a sparsifying dictionary and solves a group sparse recovery problem to locate the targets from randomly subsampled, frequency stepped SAR data.
However, all of the above techniques assume perfect knowledge of a reflective geometry of the scene, which is not necessarily feasible in practice.
U.S. 20120235849 describes a through-the-wall radar imaging system where an impulse synthetic aperture radar system transmits short, ultra-wideband (UWB) carrierless microwave pulses at an obstacle behind which a target of interest is located. The return signals are received, stored and analyzed. Portions of the return signals that represent reflections from the obstacle are identified and analyzed in the time domain to estimate the transmission coefficient of the wall, either by estimating wall parameters or by using a shift and add procedure. The estimated transmission coefficient is used to filter the received signals to reduce the components of the received signal that are generated by the obstacle, and to compensate for distortion caused by the obstacle in the portions of the transmitted signal that are reflected by the target and returned, through the obstacle, to the radar system.
U.S. 20120313810 describes a through-the wall radar apparatus that transmits a frequency modulated transmit signal having a transmit bandwidth and a receiver antenna that receives a receive signal reflected from the scene. The receive signal is mixed with the transmit signal to obtain a mixed signal. A sampling unit samples the mixed signal to obtain samples. A measurement matrix determines the positions of one or more targets of the scene by applying compressive sensing.